To continue and extend our studies on the effects of ultraviolet light, of photosensitizing dyes plus visible light and of X-rays upon the nucleic acids in vitro and in vivo; to elucidate the biochemical basis and genetic control of the repair of radiation damage in bacteria, with emphasis upon elucidating the role of different gene products in the probable branched pathways of post-replication repair and of excision repair; the interaction and/or overlap of these different repair systems, their selective inhibition by drugs, and their probable induction by radiation.